Relating to centrifugal pump impellers

ABSTRACT

A centrifugal pump impeller includes front and back shrouds and a plurality of pumping vanes therebetween, each pumping vane having a leading edge in the region of an impeller inlet and a trailing edge, the front shroud has an arcuate inner face in the region of the impeller inlet, the arcuate inner face having a radius of curvature (R s)  in the range from 0.05 to 0.16 of the outer diameter of the impeller (D 2 ) The back shroud includes an inner main face and a nose having a curved profile with a nose apex in the region of the central axis which extends towards the front shroud, there being a curved transition region between the inner main face and the nose. F r  is the radius of curvature of the transition region and the ratio F r /D 2  is from 0.32 to 0.65. Other ratios of various dimensions of the impeller are also described.

TECHNICAL FIELD

This disclosure relates generally to centrifugal pumps and moreparticularly though not exclusively to pumps for handling abrasivematerials such as for example slurries and the like.

BACKGROUND ART

Centrifugal slurry pumps, which may typically comprise hard metal orelastomer liners and/or casings that resist wear, are widely used in themining industry. Normally, the higher the slurry density, or the largeror harder the slurry particles, will result in higher wear rates andreduced pump life.

Centrifugal slurry pumps are widely used in minerals processing plantsfrom the start of the process where the slurry is very coarse withassociated high wear rates (for example, during milling), to the end ofthe process where the slurry is very much finer and the wear ratesgreatly reduced (for example, when flotation tailings are produced). Asan example, slurry pumps dealing with a coarser particulate feed dutymay only have a life of wear parts measured in weeks or months, comparedto pumps at the end of the process which have wear parts which can lastfrom one to two years in operation.

The wear in centrifugal slurry pumps that are used for handling coarseparticulate slurries typically is worst at the impeller inlet, becausethe solids have to turn through a right angle (from axial flow in theinlet pipe to radial flow in the pump impeller) and, in so doing, theparticle inertia and size results in more impacts and sliding motionagainst the impeller walls and the leading edge of the impeller vanes.

The impeller wear occurs mainly on the vanes and the front and rearshrouds at the impeller inlet. High wear in these regions can alsoinfluence the wear on the front liner of the pump. The small gap thatexists between the rotating impeller and the stationary front liner(sometimes referred to as the throatbush) will also have an effect onthe life and performance of the pump wear parts. This gap is normallyquite small, but typically increases due to wear on the impeller front,impeller shroud or due to wear on both the impeller and the front liner.

One way to reduce the flow that escapes from the high pressure casingregion of the pump (through the gap between the front of the impellerand the front liner into the pump inlet) is by incorporating a raisedand angled lip on the stationary front liner at the impeller inlet. Theimpeller has a profile to match this lip. While the flow through the gapcan be reduced by the use of expelling vanes on the front of theimpeller, the flow through the gap can also effectively minimised bydesigning and maintaining this narrow gap.

Some, but not all, pumps can have means to maintain the gap between theimpeller and the front liner as small as practicable without causingexcess wear by rubbing. A small gap normally improves the front linerlife but the wear at the impeller inlet still occurs and is notdiminished.

The high wear at the impeller entry relates to the degree of turbulencein the flow as it changes from axial to radial direction. The geometryof a poorly designed impeller and pumping vanes can dramaticallyincrease the amount of turbulence and hence wear.

The various aspects disclosed herein may be applicable to allcentrifugal slurry pumps and particularly to those that experience highwear rates at the impeller inlet or to those that are used inapplications with high slurry temperatures.

SUMMARY OF THE DISCLOSURE

In a first aspect, embodiments are disclosed of an impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud, a back shroud anda plurality of pumping vanes therebetween, each pumping vane having aleading edge in the region of an impeller inlet and a trailing edge,wherein the front shroud has an arcuate inner face in the region of theimpeller inlet, the arcuate inner face having a radius of curvature(R_(s)) in the range from 0.05 to 0.16 of the outer diameter of theimpeller (D₂), said back shroud including an inner main face and a nosehaving a curved profile with a nose apex in the region of the centralaxis which extends towards the front shroud, there being a curvedtransition region between the inner main face and the nose, whereinF_(r) is the radius of curvature of the transition region, the ratioF_(r)/D₂ being from 0.32 to 0.65.

In a second aspect, embodiments are disclosed of an impeller for use ina centrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis the impeller including a front shroud, a back shroud and aplurality of pumping vanes therebetween, each pumping vane having aleading edge in the region of an impeller inlet and a trailing edge,wherein the front shroud has an arcuate inner face in the region of theimpeller inlet, the arcuate inner face having a radius of curvature(R_(s)) in the range from 0.05 to 0.16 of the outer diameter of theimpeller (D₂), said back shroud having an inner main face and a nosehaving a curved profile with a nose apex in the region of the centralaxis which extends towards the front shroud, there being a curvedtransition region between the inner main face and the nose, whereinI_(nr) is the radius of curvature of the curved profile of the nose, theratio I_(nr)/D₂ being from 0.17 to 0.22.

In a third aspect, embodiments are disclosed of an impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis the impeller including a front shroud, a back shroud and aplurality of pumping vanes therebetween with passageways betweenadjacent pumping vanes, each pumping vane having a leading edge in theregion of an impeller inlet and a trailing edge, wherein the frontshroud has an arcuate inner face in the region of the impeller inlet,the inner face having a radius of curvature (R_(s)) in the range from0.05 to 0.16 of the outer diameter of the impeller (D₂) and wherein oneor more of the passageways have one or more discharge guide vanesassociated therewith the or each discharge guide vane being located at amain face of at least one of the shrouds.

In a fourth aspect, embodiments are disclosed of an impeller for use ina centrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud, a back shroud anda plurality of pumping vanes therebetween, each pumping vane having aleading edge in the region of an impeller inlet and a trailing edge witha main portion therebetween, wherein each pumping vane has a vaneleading edge having a radius R_(v) in the range from 0.18 to 0.19 of themain portion of the pumping vane thickness T_(v).

In a fifth aspect, embodiments are disclosed of an impeller whichincludes: a front shroud and a back shroud, the back shroud including aback face and an inner main face with an outer peripheral edge and acentral axis, a plurality of pumping vanes projecting from the innermain face of the back shroud to the front shroud, the pumping vanesbeing disposed in spaced apart relation on the inner main face providinga discharge passageway between adjacent pumping vanes, each pumping vaneincluding a leading edge portion in the region of the central axis and atrailing edge portion in the region of the peripheral edge, the backshroud further including a nose having a curved profile with a nose apexin the region of the central axis which extends towards the frontshroud, there being a curved transition region between the inner mainface and the nose, wherein I_(nr) is the radius of curvature of thecurved profile of the nose and D₂ is the diameter of the impeller, theratio I_(nr)/D₂ being from 0.02 to 0.50, wherein one or more of thepassageways have associated therewith one or more discharge guide vanesthe or each discharge guide vanes being located at a main face of atleast one of the shrouds.

In a sixth aspect, embodiments are disclosed of an impeller whichincludes: a front shroud and a back shroud, the back shroud including aback face and an inner main face with an outer peripheral edge and acentral axis, a plurality of pumping vanes projecting from the innermain face of the back shroud to the front shroud, the pumping vanesbeing disposed in spaced apart relation on the inner main face providinga discharge passageway between adjacent pumping vanes, each pumping vaneincluding a leading edge portion in the region of the central axis and atrailing edge portion in the region of the peripheral edge, the backshroud further including a nose having a curved profile with a nose apexin the region of the central axis which extends towards the frontshroud, there being a curved transition region between the inner mainface and the nose, wherein I_(nose) is the distance from a planecontaining the inner main face of the back shroud to the nose apex, atright angles to the central axis and B₂ is the pumping vane width, andthe ratio I_(nose)/B₂ being from 0.25 to 0.75, wherein one or more ofthe passageways have associated therewith one or more discharge guidevanes the or each discharge guide vanes being located at a main face ofat least one of the shrouds.

In a seventh aspect, embodiments are disclosed of an impeller whichincludes: a front shroud and a back shroud, the back shroud including aback face and an inner main face with an outer peripheral edge and acentral axis, a plurality of pumping vanes projecting from the innermain face of the back shroud to the front shroud, the pumping vanesbeing disposed in spaced apart relation on the inner main face providinga discharge passageway between adjacent pumping vanes, each pumping vaneincluding a leading edge portion in the region of the central axis and atrailing edge portion in the region of the peripheral edge, the backshroud further including a nose having a curved profile with a nose apexin the region of the central axis which extends towards the frontshroud, there being a curved transition region between the inner mainface and the nose, wherein F_(r) is the radius of curvature of thetransition region and D₂ is the diameter of the impeller, and the ratioF_(r)/D₂ being from 0.20 to 0.75, wherein one or more of the passagewayshave associated therewith one or more discharge guide vanes the or eachdischarge guide vanes being located at a main face of at least one ofthe shrouds.

In some embodiments the inner face can have a radius of curvature R_(s)in the range from 0.08 to 0.15 of the outer diameter of the impeller D₂.

In some embodiments the inner face can have a radius of curvature R_(s)in the range from 0.11 to 0.14 of the outer diameter of the impeller D₂.

In some embodiments the inner face can have a radius of curvature R_(s)in the range from 0.12 to 0.14 of the outer diameter of the impeller D₂.

In some embodiments the ratio F_(r)/D₂ can be from 0.32 to 0.65.

In some embodiments the ratio F_(r)/D₂ can be from 0.41 to 0.52.

In some embodiments the ratio I_(nr)/D₂ can be from 0.10 to 0.33.

In some embodiments the ratio I_(nr)/D₂ can be from 0.17 to 0.22.

In some embodiments I_(nose) is the distance from a plane containing theinner main face of the back shroud to the nose apex at right angles tothe central axis, and B₂ is the pumping vane width, and the ratioI_(nose)/B₂ can be from 0.25 to 0.75.

In some embodiments the ratio I_(nose)/B₂ can befrom 0.4 to 0.65.

In some embodiments the ratio I_(nose)/B₂ can be from 0.48 to 0.56.

In some embodiments the or each pumping vane can have a main portionbetween the leading and trailing edge portions thereon, the vane leadingedge portion tapered transition length and a leading edge having aradius R_(v) in the range from 0.09 to 0.45 of the thickness T_(v), of amain vane portion.

In some embodiments the leading edge of the vane can be straight butpreferably profiled to best control the inlet angle, which can varybetween the rear and front shrouds to achieve lower turbulence and wakeas the flow enters the impeller passageway. This transition region fromthe leading edge radius to the full vane thickness can be a linear orgradual transition from the radius on the leading edge (R_(v)) to themain portion thickness (T_(v)). In one embodiment, each vane can have a.transition length L_(t) between the leading edge and main portionthickness, the transition length being in the range from 0.5 T_(v) to 3T_(v), that is, the transition length varies from 0.5 to 3 times thevane thickness.

In some embodiments the vane leading edge can have a radius R_(v), inthe range from 0.125 to 0.31 of the thickness T_(v) of the main portion.

In some embodiments the vane leading edge can have a radius R_(v), inthe range from 0.18 to 0.19 of the thickness T_(v) of the main portion.

In some embodiments the thickness T_(v) of the main portion can be inthe range from 0.03 to 0.11 of the outer diameter of the impeller D₂.

In some embodiments the pumping vane thickness T_(v) of the main portioncan be in the range from 0.055 to 0.10 of the outer diameter of theimpeller D₂.

In some embodiments each vane can have a transition length L_(t) betweenthe leading edge and full vane thickness, the transition length being inthe range from 0.5 T_(v) to 3 T_(v).

In some embodiments the thickness of the main portion can besubstantially constant throughout its length.

In some embodiments each pumping vane can have a vane leading edgehaving a radius R_(v) in the range from 0.09 to 0.45 of the main portionthickness T_(v).

In some embodiments the vane leading edge can have a radius R_(v) in therange from 0.125 to 0.31 of the main portion thickness T_(v).

In some embodiments the vane leading edge can have a radius R_(v), inthe range from 0.18 to 0.19 of the main portion thickness T_(v).

In some embodiments the main portion thickness T_(v) of each vane can bein the range from 0.03 to 0.11 of the outer diameter D₂ of the impeller.

In some embodiments the main portion thickness T_(v) of each vane can bein the range from 0.055 to 0.10 of the outer diameter D₂ of theimpeller.

In some embodiments each vane can have a transition length L_(t) betweenthe leading edge and full vane thickness, the transition length being inthe range from 0.5 T_(v) to 3 T_(v).

In some embodiments one or more of the passageways can have one or moredischarge guide vanes associated therewith, the or each discharge guidevane located at the main face of at least one of the or each shroud(s).

In some embodiments the or each discharge guide vane can be a projectionfrom the main face of the shroud with which it is associated and whichextends into a respective passageway.

In some embodiments the or each discharge guide vane can be elongate.

In some embodiments the or each discharge guide vane can have an outerend adjacent the peripheral edge of the shroud, the discharge guide vaneextending inwardly and terminating at an inner end which is intermediatethe central axis and the peripheral edge of the shroud with which it isassociated.

In some embodiments two said shrouds are provided, and one or more ofthe shrouds can have a discharge guide vane projecting from a main facethereof.

In some embodiments the or each said discharge guide vane can have aheight which is from 5 to 50 percent of pumping vane width.

In some embodiments the or each discharge guide vane generally can havethe same shape and width of the main pumping vanes when viewed in ahorizontal cross-section.

In some embodiments each discharge guide vane can be of a taperingheight.

In some embodiments each discharge guide vane can be of a taperingwidth.

In some embodiments the pumping vane leading edge angle A₁ to theimpeller central axis can be from 20° to 35°.

In some embodiments the impeller inlet diameter D₁ can be in the rangefrom 0.25 to 0.75 of the impeller outer diameter D₂.

In some embodiments the impeller inlet diameter D₁ can be in the rangefrom 0.25 to 0.5 of the impeller outer diameter D₂.

In some embodiments the impeller inlet diameter D₁ can be in the rangefrom 0.40 to 0.75 of the impeller outer diameter D₂.

In an eighth aspect embodiments are disclosed of in combination, animpeller as described in any of the preceding embodiments and a frontliner, the front liner having a raised lip which subtends an angle (A₃)to the impeller central axis in the range from 10° to 80°.

In a ninth aspect embodiments are disclosed of, in combination, animpeller as described in any of the preceding embodiments and a frontliner, the front liner having an inner end and an outer end, thediameter D₄ of the inner end being in the range 0.55 to 1.1 of thediameter D₃ of the outer end.

In a tenth aspect embodiments are disclosed of, in combination, animpeller as described in any of the preceding embodiments and a frontliner, defining an angle A₂ between the parallel faces of the impellerand front liner, and a plane normal to the rotation axis which is in therange from 0° to 20°.

In an eleventh aspect embodiments are disclosed of a method ofretrofitting an impeller to a centrifugal pump, the pump including apump casing having a chamber therein, an inlet for delivering materialto be pumped to the chamber and an outlet for discharging material fromthe chamber, the impeller being mounted for rotation within the chamberwhen in use about a rotation axis the impeller being as described in anyof the preceding embodiments, the method including operativelyconnecting the impeller to a drive shaft of a drive which extends intothe chamber.

In some embodiments an impeller or an impeller and liner combination mayinclude a combination of any two or more of the aspects of certainembodiments described above.

To minimise the turbulence in the impeller inlet region, the arrangementdesirably incorporates features to minimise the cavitationcharacteristics on the performance of the pump. This means that thedesign minimises the net positive intake (or suction) head required(normally called NPSH). Cavitation occurs when the pressure available atthe pump intake is lower than that required by the pump, causing theslurry water to ‘boil’ and vapour pockets, wakes and turbulence to becreated. The vapour and turbulence will cause damage to the pump inletvanes and shrouds by removing material and creating pinholes and smallpockets of wear that can increase in size with time.

The slurry particles entering the inlet can be deflected from a smoothstreamline by the vapour and turbulent flow, thereby accelerating therate of wear. A turbulent flow creates small to large scale spirallingor vortex types of flow patterns. When the particles are trapped inthese spiralling flows, their velocity is greatly increased and, as ageneral rule, the wear on the pump parts tends to increase. The wearrate in slurry pumps can be related to the particle velocity raised tothe power of two to three, so maintaining low particle velocities isuseful to minimise wear.

Some mineral processing plants (such as alumina production plants)require elevated operating temperatures to assist with the mineralextraction process. High temperature slurries require pumps that havegood cavitation-damping characteristics. The lower the NPSH required bythe pump, the better the pump will be able to maintain its performance.An impeller design having low cavitation characteristics will assist inboth minimising wear and in minimising the effect on the pumpperformance, and therefore minerals processing plant output.

One of the ways to decrease turbulence in the feed slurry entering thepump is to provide a smooth change in angle for the slurry flow and itsentrained particles, as the slurry moves from a horizontal to a verticaldirection of flow. The inlet may be rounded by contouring the internalpassageway shape of the impeller in conjunction with the front liner.The rounding produces more streamlined flow and less turbulence as aresult. The inlet of the front liner can also be rounded or incorporatea smaller inlet diameter or throat which can also assist in smoothingthe turning flow path of the slurry.

A further means to turn the flow more evenly is to incorporate an angledfront liner and matching angled impeller front face.

Lower rates of turbulence at the impeller inlet region will result inless wear overall. Wear life is of primary importance for pumps in heavyand severe slurry applications in the minerals processing industries. Asdescribed hereinabove, to achieve lower wear at the impeller inletrequires a combination of certain dimensional ratios to produce specificlow turbulence geometry. The inventors have surprisingly discovered thatthis preferred geometry is largely independent of the ratio of theimpeller outside diameter to the inlet diameter (normally referred to asthe impeller ratio).

It has been discovered that the various ratios described above or incombination provide an optimum geometry to firstly produce a smooth flowpattern and to minimise the shock losses at the entrance to the impellerpassageway and secondly to control the amount of turbulence for as longas possible through the impeller passageway. The various ratios areimportant because these control the flow from an axial direction intothe impeller through a turn of ninety degrees to form a radial flow, andalso to smooth the flow past the leading edges of the main pumping vanesinto each of the impeller discharge passageways (that is, thepassageways between each of the main pumping vanes).

In particular, an impeller having the dimensional ratios of R_(s)/D₂ inthe range from 0.05 to 0.16, and F_(r)/D₂ from 0.32 to 0.65 have beenfound to provide the advantageous effects described above.

In particular, an impeller having the dimensional ratios of R_(s)/D₂ inthe range from 0.05 to 0.16, and I_(nr)/D₂ from 0.17 to 0.22 have beenfound to provide the advantageous effects described above.

In particular, an impeller having pumping vanes with the dimensionalratios of R_(v)/T_(v) in the range from 0.18 to 0.19 have been found toprovide the advantageous effects described above.

Further improvement was also achieved by the provision of dischargeguide vanes, as described above. The discharge guide vanes are believedto control the turbulence due to vortices in the flow of material whichis passing through the impeller passageway during use. Increasedturbulence can lead to increased wear of impeller and volute surfaces aswell as increased energy losses, which ultimately require an operator toinput more energy into the pump to achieve a desired throughput.Depending on the selected position of the discharge guide vanes, theturbulence region immediately in front of the pumping face of theimpeller pumping vanes can be substantially confined. As a result, theintensity (or strength) of the vortices is diminished because they arenot allowed to grow in an unconstrained manner. A further beneficialoutcome was that the smoother flow throughout the impeller passagewayreduced the turbulence and thereby also reduced the wear due toparticles in the slurry flow.

The improvements in performance included that the pressure generated bythe pump gave less depression at higher flows (that is, less loss ofenergy with flow-noting that traditional impellers have a steepercharacteristic loss with same number of main pumping vanes); that theefficiency increased 7 to 8% in absolute terms; that the cavitationcharacteristic of the pump reduced and remained flatter, right out tohigher flows (conventional impellers have a steeper characteristic); andthat the wear life of the impeller increased by 50% compared to atraditional design of impeller.

Under current, traditional design protocols it was always consideredthat one performance parameter could be increased but at the expense ofanother eg higher efficiency but lower wear life. The present inventionhas contradicted this view by achieving all round better performance forall parameters.

As a result of an all round better performance, the impeller can bemanufactured using ‘standard’ materials, without the need for specialalloys materials which would otherwise be required to solve localisedhigh wear issues.

Experimental trials have demonstrated that these design parameters andthe specification of certain dimensional ratios can produce relativelylow or substantially optimum impeller wear, especially around the eye(inlet region) of the impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of theapparatus, and method as set forth in the Summary, specific embodimentsof the method and apparatus will now be described, by way of example,and with reference to the accompanying drawings in which:

FIG. 1 illustrates an exemplary, schematic, partial cross-sectional sideelevation of a pump incorporating an impeller and an impeller and linercombination, in accordance with one embodiment;

FIG. 1A illustrates a detailed view of a portion of the impeller of FIG.1; FIG. 2 illustrates an exemplary, schematic, cross-sectional top viewof an impeller pumping vane in accordance with another embodiment; and

FIGS. 3 to 12 illustrate exemplary whole and partially sectional viewsof an impeller and of an inlet liner, with some views showing thecombination of impeller and inlet liner in accordance with certainembodiments.

FIG. 13A illustrates an exemplary, schematic, cross-sectional sideelevation of an impeller and liner combination, in accordance with oneembodiment showing the various regions of liner inlet (1), impellerfront shroud (2), impeller front shroud outlet (3), and impeller backshroud nose (4).

FIG. 13B illustrates an exemplary, schematic, cross-sectional sideelevation of an impeller and liner combination, in accordance with oneembodiment wherein the data points are produced by curve fitting andlinear regression modelling to show the internal profile of the variousregions shown in FIG. 13A.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to FIGS. 1 and 1A there is illustrated an exemplary pump 10 inaccordance with certain embodiments including a pump casing 12, a backliner 14, a front liner 30 and a pump outlet 18. An internal chamber 20is adapted to receive an impeller 40 for rotation about rotational axisX-X.

The front liner 30 includes a cylindrically-shaped delivery section 32through which slurry enters the pump chamber 20. The delivery section 32has a passage 33 therein with a first, outermost end 34 operativelyconnectable to a feed pipe (not shown) and a second, innermost end 35adjacent the chamber 20. The front liner 30 further includes a side wallsection 15 which mates with the pump casing 12 to form and enclose thechamber 20, the side wall section 15 having an inner face 37. The secondend 35 of the front liner 30 has a raised lip 38 thereat, which isarranged to mate with the impeller 40.

The impeller 40 includes a hub 41 from which a plurality ofcircumferentially spaced pumping vanes 42 extend. An eye portion 47extends forwardly from the hub towards the passage 33 in the frontliner. The pumping vanes 42 include a leading edge 43 located at theregion of the impeller inlet 48, and a trailing edge 44 located at theregion of the impeller outlet 49. The impeller further includes a frontshroud 50 and a back shroud 51, the vanes 42 being disposedtherebetween.

In the particular embodiment of a partial impeller 10A shown in FIG. 2,one exemplary pumping vane 42 only is shown which extends between theopposing main inner faces of the shrouds 50, 51. Normally such animpeller 10A has a plurality of such pumping vanes spaced evenly aroundthe area between the said shrouds 50, 51, for example three, four orfive pumping vanes are usual in slurry pumps. In this drawing only onepumping vane has been shown for convenience to illustrate the features.As shown in FIG. 2 the exemplary pumping vane 42 is generally arcuate incross-section and includes an inner leading edge 43 and an outertrailing edge 44 and opposed side faces 45 and 46, the side face 45being a pumping or pressure side. The vanes are normally referred to asbackward-curving vanes when viewed with the direction of rotation.Reference numerals identifying the various features described above haveonly been indicated on the one vanes 42 shown, for the sake of clarity.The important major dimensions of L_(t), R_(v) and T_(v) have been shownin the Figure and are defined below in this specification.

In accordance with certain embodiments, an exemplary impeller isillustrated in FIGS. 3 to 12. For convenience the same referencenumerals have now been used to identify the same parts described withreference to FIGS. 1, 1A and 2. In the particular embodiment shown inFIGS. 3 to 12, the impeller 40 has a plurality of discharge guide vanes(or vanelets). The discharge guide vanes are in the form of elongate,flat-topped projections 55 which are generally sausage-shaped incross-section. These projections 55, extend respectively from the mainface of the back shroud 51 and are arranged in between two adjacentpumping vanes 42. The projections 55 have a respective outer end 58which is located adjacent to the outer peripheral edge the shroud 51 onwhich they are disposed. The discharge guide vanes also have an innerend 60, which is located somewhere midway a respective passageway. Theinner ends 60, of respective discharge guide vanes 55 are spaced somedistance from the central rotational axis X-X of the impeller 40.Typically although not necessarily, the discharge guide vanes can beassociated with each passageway.

Each discharge guide vane in the form of a projection 55 is shown in thedrawings with a height of approximately 30-35% of the width of thepumping vane 42 where the width of the pumping vane is defined as thedistance between the front and back shrouds of the impeller. In furtherembodiments the guide vane height can be between 5% to 50% of the saidpumping vane 42 width. Each guide vane is of generally constant heightalong its length, although in other embodiments the guide vane can betapered in height and also tapered in width. As is apparent from thedrawings, the vanes have bevelled peripheral edges.

In the embodiment shown in FIGS. 3 to 12, each discharge guide vane canbe located closer to the pumping or pressure side face of the closestadjacent pumping vane. The positioning of a discharge guide vane closerto one adjacent pumping vane can advantageously improve pumpperformance. Such embodiments are also disclosed in this Applicant'sco-pending application entitled “Slurry Pump Impeller” which was filedon the same day as the present application, the contents of which areincluded herein by way of cross-reference.

In still other embodiments, the discharge guide vanes can extend for ashorter or longer distance into the discharge passageway than is shownin the embodiments of FIGS. 3 to 12, depending on the fluid or slurry tobe pumped.

In still other embodiments, there can be more than one discharge guidevane per shroud inner main face, or in some instances no discharge guidevane on one of the opposing inner main faces of any two shrouds whichdefine a discharge passageway.

In still other embodiments, the discharge guide vanes can be of adifferent cross-sectional width to the main pumping vanes, and may noteven necessarily be elongate, so long as the desired effect on the flowof slurry at the impeller discharge is achieved.

It is believed that the discharge guide vanes will reduce the potentialfor high-velocity vortex type flows to form at low flows. This reducesthe potential for particles to wear into the front or rear shroudsthereby resulting in wear cavities in which vortex type flows couldoriginate and develop. The guide vanes will also reduce the mixing ofthe split off flow regions at the immediate exit of the impeller intothe already rotating flow pattern in the volute. It is felt that thedischarge guide vanes will smooth and reduce the turbulence of the flowfrom the impeller into the pump casing or volute.

The impeller 10 further includes expeller, or auxiliary, vanes 67, 68,69 on respective outer faces of the shrouds. Some of the vanes on theback shroud 67, 68 have different widths. As shown in the Figures, allvanes including the discharge guide vanes have bevelled edges.

FIGS. 1 and 2 of the drawings identify the following parameters:

-   -   D₁ Impeller inlet diameter at the intersection point of the        front shroud and leading edge of the pumping vane    -   D₂ Impeller outside diameter which is the outer diameter of the        pumping vanes which in some exemplary embodiments is the same as        the impeller back shroud.    -   D₃ Front liner first end diameter    -   D₄ Front liner second end diameter    -   A₁ Angle between vane leading edge and impeller central rotation        axis    -   A₂ Angle between the parallel faces of impeller and front liner,        and a plane normal to the rotation axis    -   A₃ Angle of front liner raised lip away from the impeller        central rotational axis    -   R_(s) Impeller front shroud radius of curvature at that point        where the throat bush and the front shroud of the impeller are        aligned (that is, where the flow leaves the throat bush and        enters the impeller)    -   R_(v) Vane leading edge radius    -   T_(v) Vane thickness of pumping vane main portion    -   L_(t) Transition length of vane    -   B₂ Impeller outlet width    -   I_(nr) Radius of curvature of the curved profile of the nose of        the impeller at the hub    -   I_(nose) Distance from a plane containing the inner main face of        the back shroud to the nose apex, at right angles to the central        axis    -   F_(r) Radius of curvature of the transition region between the        inner main face and the nose.

Preferably one or more of these parameters have dimensional ratios inthe following ranges:

D₄ = 0.55  D₃  to  1.1  D₃ $\begin{matrix}{D_{1} = {0.25\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.75\mspace{14mu} D_{2}\mspace{14mu} {more}\mspace{14mu} {preferably}}} \\{{0.25\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.5\mspace{14mu} D_{2}\mspace{14mu} {more}\mspace{14mu} {preferably}}} \\{{0.40\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.75\mspace{14mu} {D_{2}.}}}\end{matrix}$ $\begin{matrix}{{R_{S} = {0.05\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.16\mspace{14mu} D_{2}}},{{more}\mspace{14mu} {preferably}}} \\{{{0.08\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.15\mspace{14mu} D_{2}},{{more}\mspace{14mu} {preferably}}}} \\{{0.11\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.14\mspace{14mu} D_{2}}}\end{matrix}$ $\begin{matrix}{{R_{V} = {0.09\mspace{14mu} T_{V}\mspace{14mu} {to}\mspace{14mu} 0.45\mspace{14mu} T_{V}}},{{more}\mspace{14mu} {preferably}}} \\{{{0.125\mspace{14mu} T_{V}\mspace{14mu} {to}\mspace{14mu} 0.31\mspace{14mu} T_{V}},{{more}\mspace{14mu} {preferably}}}} \\{{0.18\mspace{14mu} T_{V}\mspace{14mu} {to}\mspace{14mu} 0.19\mspace{14mu} T_{V}}}\end{matrix}$ $\begin{matrix}{T_{V} = {0.03\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.11\mspace{14mu} D_{2}\mspace{14mu} {more}\mspace{14mu} {preferably}}} \\{{0.055\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.10\mspace{14mu} D_{2}}}\end{matrix}$ L_(t) = 0.5  T_(v)  to  3T_(v)B₂ = 0.08  D₂  to  0.2  D₂ $\begin{matrix}{{I_{nr} = {0.02\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.50\mspace{14mu} D_{2}}},{{more}\mspace{14mu} {preferably}}} \\{{= {0.10\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.33\mspace{14mu} D_{2}}},{{more}\mspace{14mu} {preferably}}} \\{= {0.17\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.22\mspace{14mu} D_{2}}}\end{matrix}$ $\begin{matrix}{{I_{nose} = {0.25\mspace{14mu} B_{2}\mspace{14mu} {to}\mspace{14mu} 0.75\mspace{14mu} B_{2}}},{{more}\mspace{14mu} {preferably}}} \\{{= {0.40\mspace{14mu} B_{2}\mspace{14mu} {to}\mspace{14mu} 0.65\mspace{14mu} B_{2}}},{{more}\mspace{14mu} {preferably}}} \\{= {0.48\mspace{14mu} B_{2}\mspace{14mu} {to}\mspace{14mu} 0.56\mspace{14mu} B_{2}}}\end{matrix}$ $\begin{matrix}{{F_{r} = {0.20\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.75\mspace{14mu} D_{2}}},{{more}\mspace{14mu} {preferably}}} \\{{= {0.32\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.65\mspace{14mu} D_{2}}},{{more}\mspace{14mu} {preferably}}} \\{= {0.41\mspace{14mu} D_{2}\mspace{14mu} {to}\mspace{14mu} 0.52\mspace{14mu} {D_{2}.}}}\end{matrix}$

And have angles in the ranges:

-   -   A₂=0 to 20°    -   A₃=10° to 80°    -   A₁=20° to 35°

EXAMPLES

Comparative trials were conducted with a conventional pump and a pumpaccording an exemplary embodiment. The various relevant dimensions ofthe two pumps are set out below.

Conventional Pump Impeller New Pump Impeller D₁ = 203 mm = 226 mm D₂ =511 mm = 550 mm R_(S) = 156 mm = 60 mm R_(v) = 2 mm = 6 mm T_(v) =Varies (up to maximum of 76 mm) = 32 mm L_(t) = None = 67 mm B₂ = 76 mm= 72 mm F_(r) = 232 mm = 228 mm I_(nr) = 95 mm = 95 mm A_(l) = 0(parallel to inlet axis) = 25°

Front Liner Front Liner A₂ = 0 (perpendicular to inlet axis) = ditto A₃= 60° = 60° D₃ = 203 mm = 203 mm D₄ = 200 mm = 224 mmFor the exemplary New Pump Impeller described herein above, the ratioR_(s)/D₂ is 0.109; the ratio F_(r)/D₂ is 0.415; the ratio I_(nr)/D₂ is0.173 and the ration R_(v)/T_(v) is 0.188.

Example 1

Both the new and conventional pumps were run at the same duty flow andspeed on a gold mining ore. The conventional pump impeller life was1,600 to 1,700 hours and front liner life 700 to 900 hours. The newdesign impeller and front liner life were both 2,138 hours.

Example 2

Both the new and conventional pumps were run at the same duty flow andspeed on a gold mining ore which results in rapid wear due to the highsilicon sand content of the slurry. Following three trials, the newimpeller and front liner showed consistently 1.4 to 1.6 times more lifethan the conventional metal parts in the same material.

The conventional impeller typically failed by gross wear on the pumpvanes and holing of the backshroud. The new impeller showed very littleof this same type of wear.

Example 3

Both the new and conventional pumps were run at the same duty flow andspeed in an alumina refinery in a duty which was critical to providingthe proper feed to the plant. This duty was at high temperature and sofavoured an impeller design with low cavitation characteristics.

The average life of the conventional impeller and front liner was 4,875hours with some impeller wear, but typically the front liner failed byholing during use.

The new impeller and front liner life were in excess of 6,000 hours andwithout holing.

Example 4

Both the new and conventional pumps were run at the same duty flow andspeed in an aluminia refinery where pipe and tank scaling can affect theproduction rate of the pump due to the effects of cavitation.

Based on the experiment, it has been calculated that the new impellerand front liner allowed an additional 12.5% increase in throughput whilestill remaining unaffected by cavitation.

EXPERIMENTAL SIMULATION

Computational experiments were carried out to define equations for thevarious designs of impeller disclosed herein, using commercial software.This software applies normalised linear regression or curve fittingmethods to define a polynomial which describes the curvature of theinner faces of the impeller shrouds for certain embodiments disclosedherein.

Each selected embodiment of an impeller when viewed in cross-section ina plane drawn through the rotational axis has four general profileregions which each have distinct features of shape, as illustrated inFIG. 13A. FIG. 13B is the profile of the features of shape of aparticular impeller which have been produced by use of the polynomial.Along the X-axis (which is a line which extends from the hub of theimpeller through the centre of the impeller nose and coaxial with therotational axis X-X), actual impeller dimensions are taken and dividedby B₂ (the impeller outlet width) to produce a normalised value X_(n).Along the Y-axis (which is a line which extends at right angles to therotational axis X-X and in the plane of the main inner face of the backshroud), actual impeller dimensions are taken and divided by 0.5×D₂(half of the impeller outside diameter) to produce a normalised valueY_(n). The values of X_(n) and Y_(n) are then regressed to calculate apolynomial to describe the profile of the region (2) which is the acuateinner face in the region of the impeller inlet, and the profile of theregion (4) which is the curved profile of the impeller nose region.

In one embodiment where D₂ is 550 mm and B₂ is 72 mm, the profile region(2) is defined by:

y _(n)=−2.3890009903x _(n) ⁵+19.4786939775x _(n) ⁴−63.2754154980x _(n)³+102.6199259524x _(n) ²−83.4315403428x+27.7322233171

In one embodiment where D₂ is 550 mm and B₂ is 72 mm, the profile region(4) is defined by:

y=−87.6924201323x _(n) ⁵+119.7707929717x _(n) ⁴−62.3921978066x _(n)³+16.0543468684x _(n) ²−2.7669594052x+0.5250083657.

In one embodiment where D₂ is 1560 mm and B₂ is 190 mm, the profileregion (2) is defined by:

y _(n)=−7.0660920862x _(n) ⁵+56.8379443295x _(n) ⁴−181.1145997000x _(n)³+285.9370452104x _(n) ²−223.9802206897x+70.2463717260

In one embodiment where D₂ is 1560 mm and B₂ is 190 mm, the profileregion (4) is defined by:

y _(n)=−52.6890959578x _(n) ⁵+79.4531495101x _(n) ⁴−45.7492175031x _(n)³+13.0713205894x _(n) ²−2.5389732284x+0.5439201928.

In one embodiment where D₂ is 712 mm and B₂ is 82 mm, the profile region(2) is defined by:

y _(n)=−0.8710521204x _(n) ⁵+7.8018806610x _(n) ⁴−27.9106218350x _(n)³+50.0122747105x _(n) ²−45.1312740213x+16.9014790579

In one embodiment where D₂ is 712 mm and B₂ is 82 mm, the profile region(4) is defined by:

y _(n)=−66.6742503139x _(n) ⁵+103.3169809752x _(n) ⁴−60.6233286019x _(n)³+17.0989215719x _(n) ²−2.9560300900x+0.5424661895.

In one embodiment where D₂ is 776 mm and B₂ is 98 mm, the profile region(2) is defined by:

y_(n)=−0.2556639974x _(n) ⁵+2.6009971578x _(n) ⁴−10.5476726720x _(n)³+21.4251116716x _(n) ²−21.9586498788x+9.5486465528

In one embodiment where D₂ is 776 mm and B₂ is 98 mm, the profile region(4) is defined by:

y _(n)=−74.2097253182x _(n) ⁵+115.5559502836x _(n) ⁴−67.8953477381x _(n)³+19.1100516593x _(n) ²−3.2725057764x+0.5878323997.

In the foregoing description of certain exemplary embodiments, specificterminology has been resorted to for the sake of clarity. However, theinvention is not intended to be limited to the specific terms soselected, and it is to be understood that each specific term includesall technical equivalents which operate in a similar manner toaccomplish a similar technical purpose. Terms such as “front” and“rear”, “above” and “below” and the like are used as words ofconvenience to provide reference points and are not to be construed aslimiting terms.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that that prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

Finally, it is to be understood that various alterations, modificationsand/or additions may be incorporated into the various constructions andarrangements of parts without departing from the spirit or ambit of theinvention.

1. An impeller for use in a centrifugal pump, the pump including a pumpcasing having a chamber therein, an inlet for delivering material to bepumped to the chamber and an outlet for discharging material from thechamber, the impeller being mounted for rotation within the chamber whenin use about a rotation axis, the impeller including a front shroud, aback shroud and a plurality of pumping vanes therebetween, each pumpingvane having a leading edge in the region of an impeller inlet and atrailing edge, wherein the front shroud has an arcuate inner face in theregion of the impeller inlet, the arcuate inner face having a radius ofcurvature (R_(s)) in the range from 0.05 to 0.16 of the outer diameterof the impeller (D₂), said back shroud including an inner main face anda nose having a curved profile with a nose apex in the region of thecentral axis which extends towards the front shroud, there being acurved transition region between the inner main face and the nose,wherein F_(r) is the radius of curvature of the transition region, theratio F_(r)/D₂ being from 0.32 to 0.65.
 2. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud, a back shroud anda plurality of pumping vanes therebetween, each pumping vane having aleading edge in the region of an impeller inlet and a trailing edge,wherein the front shroud has an arcuate inner face in the region of theimpeller inlet, the arcuate inner face having a radius of curvature(R_(s)) in the range from 0.05 to 0.16 of the outer diameter of theimpeller (D₂), said back shroud having an inner main face and a nosehaving a curved profile with a nose apex in the region of the centralaxis which extends towards the front shroud, there being a curvedtransition region between the inner main face and the nose, whereinI_(nr) is the radius of curvature of the curved profile of the nose, theratio I_(nr)/D₂ being from 0.17 to 0.22.
 3. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud, a back shroud anda plurality of pumping vanes therebetween with passageways betweenadjacent pumping vanes, each pumping vane having a leading edge in theregion of an impeller inlet and a trailing edge, wherein the frontshroud has an arcuate inner face in the region of the impeller inlet,the inner face having a radius of curvature (R_(s)) in the range from0.05 to 0.16 of the outer diameter of the impeller (D₂) and wherein oneor more of the passageways have one or more discharge guide vanesassociated therewith, the or each discharge guide vane being located ata main face of at least one of the shrouds.
 4. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud, a back shroud anda plurality of pumping vanes therebetween, each pumping vane having aleading edge in the region of an impeller inlet and a trailing edge witha main portion therebetween, wherein each pumping vane has a vaneleading edge having a radius R_(v) in the range from 0.18 to 0.19 of thepumping vane thickness T_(v) of the main portion thereof.
 5. An impellerwhich includes a front shroud and a back shroud, the back shroudincluding a back face and an inner main face with an outer peripheraledge and a central axis, a plurality of pumping vanes projecting fromthe inner main face of the back shroud to the front shroud, the pumpingvanes being disposed in spaced apart relation on the inner main faceproviding a discharge passageway between adjacent pumping vanes, eachpumping vane including a leading edge portion in the region of thecentral axis and a trailing edge portion in the region of the peripheraledge, the back shroud further including a nose having a curved profilewith a nose apex in the region of the central axis which extends towardsthe front shroud, there being a curved transition region between theinner main face and the nose, wherein I_(n) is the radius of curvatureof the curved profile of the nose and D₂ is the diameter of theimpeller, the ratio I_(nr)/D₂ being from 0.02 to 0.5, wherein one ormore of the passageways have associated therewith one or more dischargeguide vanes the or each discharge guide vanes being located at a mainface of at least one of the shrouds.
 6. An impeller which includes afront shroud and a back shroud, the back shroud including a back faceand an inner main face with an outer peripheral edge and a central axis,a plurality of pumping vanes projecting from the inner main face of theback shroud to the front shroud, the pumping vanes being disposed inspaced apart relation on the inner main face providing a dischargepassageway between adjacent pumping vanes, each pumping vane including aleading edge portion in the region of the central axis and a trailingedge portion in the region of the peripheral edge, the back shroudfurther including a nose having a curved profile with a nose apex in theregion of the central axis which extends towards the front shroud, therebeing a curved transition region between the inner main face and thenose, wherein I_(nose) is the distance from a plane containing the innermain face of the back shroud to the nose apex at right angles to thecentral axis, and B₂ is the pumping vane width, and the ratioI_(nose)/B₂ being from 0.25 to 0.75, wherein one or more of thepassageways have associated therewith one or more discharge guide vanesthe or each discharge guide vanes being located at a main face of atleast one of the shrouds.
 7. An impeller which includes a front shroudand a back shroud, the back shroud including a back face and an innermain face with an outer peripheral edge and a central axis, a pluralityof pumping vanes projecting from the inner main face of the back shroudto the front shroud, the pumping vanes being disposed in spaced apartrelation on the inner main face providing a discharge passageway betweenadjacent pumping vanes, each pumping vane including a leading edgeportion in the region of the central axis and a trailing edge portion inthe region of the peripheral edge, the back shroud further including anose having a curved profile with a nose apex in the region of thecentral axis which extends towards the front shroud, there being acurved transition region between the inner main face and the nose,wherein F_(r) is the radius of curvature of the transition region and D₂is the diameter of the impeller, and the ratio F_(r)/D₂ being from 0.20to 0.75, wherein one or more of the passageways have associatedtherewith one or more discharge guide vanes the or each discharge guidevanes being located at a main face of at least one of the shrouds.
 8. Animpeller as claimed in claim 1 or claim 3, wherein the inner face has aradius of curvature R_(s) in the range from 0.08 to 0.15 of the outerdiameter of the impeller D₂.
 9. An impeller as claimed in claim 1 orclaim 3, wherein the inner face has a radius of curvature R_(s) in therange from 0.11 to 0.14 of the outer diameter of the impeller D₂.
 10. Animpeller as claimed in claim 1 or claim 3, wherein the inner face has aradius of curvature R_(s) in the range from 0.12 to 0.14 of the outerdiameter of the impeller D₂.
 11. An impeller according to claim 1 orclaim 7 wherein the ratio F_(r)/D₂ is from 0.41 to 0.52.
 12. An impelleraccording to claim 2 or claim 5 wherein the ratio I_(nr)/D₂ is from 0.10to 0.33.
 13. An impeller according to claim 2 or claim 5 wherein theratio I_(nr)/D₂ is from 0.17 to 0.22.
 14. An impeller according to anyone of claims 1 to 5 or claims 7 to 13 wherein L_(nose) is the distancefrom a plane containing the inner main face of the back shroud to thenose apex at right angles to the central axis, and B₂ is the pumpingvane width, and the ratio I_(nose)/B₂ being from 0.25 to 0.75.
 15. Animpeller according to claim 14 wherein the ratio I_(nose)/B₂ is from 0.4to 0.65.
 16. An impeller according to claim 14, wherein the ratioI_(nose)/B₂ is from 0.48 to 0.56.
 17. An impeller according to any oneof claims 1 to 3 or 5 to 16 wherein each pumping vane has a main portionbetween the leading and trailing edge portions thereon, the vane leadingedge portion tapered transition length and a leading edge having aradius R_(v) in the range from 0.09 to 0.45 of the thickness T_(v) of amain vane portion.
 18. An impeller as claimed in claim 17, wherein thevane leading edge has a radius R_(v) in the range from 0.125 to 0.31 ofthe thickness T_(v) of the main portion.
 19. An impeller as claimed inclaim 17 or claim 18, wherein the vane leading edge has a radius R_(v)in the range from 0.18 to 0.19 of the thickness T_(v) of the mainportion.
 20. An impeller according to any one of claims 4 or 17 to 19,wherein the thickness T_(v) of the main portion is in the range from0.03 to 0.11 of the outer diameter of the impeller D₂.
 21. An impelleraccording to claim 20, wherein the pumping vane thickness T_(v) of themain portion is in the range from 0.055 to 0.10 of the outer diameter ofthe impeller D₂.
 22. An impeller according to any one of claims 4 or 17to 21, wherein each vane has a transition length L_(t) between theleading edge and full vane thickness, the transition length being in therange from 0.5 T_(v) to 3 T_(v).
 23. An impeller according to any one ofclaims 4 or 17 to 22 wherein the thickness of the main portion issubstantially constant throughout its length.
 24. An impeller accordingto any one of claim 1 to 3 or 5 wherein each pumping vane has a vaneleading edge having a radius R_(v) in the range from 0.09 to 0.45 of themain portion thickness T_(v).
 25. An impeller as claimed in claim 24,wherein the vane leading edge has a radius R_(v) in the range from 0.125to 0.31 of the main portion thickness T_(v).
 26. An impeller as claimedin claim 24 or claim 25, wherein the vane leading edge has a radiusR_(v) in the range from 0.18 to 0.19 of the main portion thicknessT_(v).
 27. An impeller according to any one of claims 24 to 26, whereinthe main portion thickness T_(v) of each vane is in the range from 0.03to 0.11 of the outer diameter D₂ of the impeller.
 28. An impelleraccording to claim 27, wherein the main portion thickness T_(v) of eachvane is in the range from 0.055 to 0.10 of the outer diameter D₂ of theimpeller.
 29. An impeller according to any one of claims 24 to 28,wherein each vane has a transition length L_(t) between the leading edgeand full vane thickness, the transition length being in the range from0.5 T_(v) to 3T_(v).
 30. An impeller according to any one of claims 1,2, or any one of claims 8 to 29 when dependent on claim 1 or claim 2,wherein one or more of the passageways have one or more discharge guidevanes associated therewith, the or each discharge guide vane located atthe main face of at least one of the or each shroud(s).
 31. An impelleraccording to any one of claims 3, 5, 6, 7 or 30 wherein the or eachdischarge guide vane is a projection from the main face of the shroudwith which it is associated and which extends into a respectivepassageway.
 32. An impeller according to claim 30 or claim 31 whereinthe or each discharge guide vane is elongate.
 33. An impeller accordingto claim 32 wherein the or each discharge guide vane has an outer endadjacent the peripheral edge of the shroud, the discharge guide vaneextending inwardly and terminating at an inner end which is intermediatethe central axis and the peripheral edge of the shroud with which it isassociated.
 34. An impeller according to any one of claims 30 to 32wherein each said shroud has a said discharge guide vane projecting froma main face thereof.
 35. An impeller according to any one of claims 30to 34 wherein each said discharge guide vane has a height which is from5 to 50 percent of pumping vane width.
 36. An impeller according to anyone of claims 30 to 35 wherein the or each discharge guide vanegenerally has the same shape and width of the main pumping vanes whenviewed in a horizontal cross-section.
 37. An impeller according to anyone of claims 30 to 36 wherein each discharge guide vane is of atapering height.
 38. An impeller according to any one of claims 30 to 37wherein each discharge guide vane is of a tapering width.
 39. Animpeller according to any one of the preceding claims, wherein thepumping vane leading edge angle A₁ to the impeller central axis is from20° to 35°.
 40. An impeller according to any one of the precedingclaims, wherein the impeller inlet diameter D₁ is in the range from 0.25to 0.75 of the impeller outer diameter D₂.
 41. In combination, animpeller according to any one of the preceding claims and a front liner,the front liner having a raised lip which subtends an angle (A₃) to theimpeller central axis in the range from 10° to 80°.
 42. In combination,an impeller according to any one of the preceding claims and a frontliner, the front liner having an inner end and an outer end, thediameter D₄ of the inner end being in the range 0.55 to 1.1 of thediameter D₃ of the outer end.
 43. In combination, an impeller accordingto any one of the preceding claims and a front liner, defining an angleA₂ between the parallel faces of the impeller and front liner, and aplane normal to the rotation axis which is in the range from 0° to 20°.44. A method of retrofitting an impeller to a centrifugal pump, the pumpincluding a pump casing having a chamber therein, an inlet fordelivering material to be pumped to the chamber and an outlet fordischarging material from the chamber, the impeller being mounted forrotation within the chamber when in use about a rotation axis theimpeller being in accordance with any one of the preceding claims, themethod including operatively connecting the impeller to a drive shaft ofa drive which extends into the chamber.
 45. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud and a back shroud,each having a main inner face in a plane substantially at right anglesto the rotation axis and a plurality of pumping vanes therebetween, eachpumping vane having a leading edge in the region of an impeller inletand a trailing edge, wherein the front shroud has an arcuate inner facein the region of the impeller inlet, the arcuate inner face has aprofile defined by the following:y _(n)=−2.3890009903x _(n) ⁵+19.4786939775x _(n) ⁴−63.2754154980x _(n)³+102.6199259524x _(n) ²−83.4315403428x+27.7322233171 where the y_(n)axis is in the plane of the back shroud main inner face and the x_(n)axis is coaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) andx_(n) equals x/B₂, wherein x and y define the actual coordinates of animpeller front shroud arcuate inner face, and D₂ (which is the impelleroutside diameter) is 550 mm, and B₂ (which is the impeller outlet width)is 72 mm.
 46. An impeller for use in a centrifugal pump, the pumpincluding a pump casing having a chamber therein, an inlet fordelivering material to be pumped to the chamber and an outlet fordischarging material from the chamber, the impeller being mounted forrotation within the chamber when in use about a rotation axis, theimpeller including a front shroud and a back shroud, each having a maininner face in a plane substantially at right angles to the rotation axisand a plurality of pumping vanes therebetween, each pumping vane havinga leading edge in the region of an impeller inlet and a trailing edge,wherein the back shroud further includes a nose having a curved profilewith a nose apex in the region of the rotation axis which extendstowards the front shroud wherein the curved profile is defined by thefollowing:y=−87.6924201323x _(n) ⁵+119.7707929717x _(n) ⁴−62.3921978066x _(n)³+16.0543468684x _(n) ²−2.7669594052x+0.5250083657 where the y_(n) axisis in the plane of the back shroud main inner face and the x_(n) axis iscoaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) and x_(n)equals x/B₂, wherein x and y define the actual coordinates of animpeller back shroud further including a nose having a curved profile,and D₂ (which is the impeller outside diameter) is 550 mm, and B₂ (whichis the impeller outlet width) is 72 mm.
 47. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud and a back shroud,each having a main inner face in a plane substantially at right anglesto the rotation axis and a plurality of pumping vanes therebetween, eachpumping vane having a leading edge in the region of an impeller inletand a trailing edge, wherein the front shroud has an arcuate inner facein the region of the impeller inlet, the arcuate inner face has aprofile defined by the following:y _(n)=−7.0660920862x _(n) ⁵+56.8379443295x _(n) ⁴−181.1145997000x _(n)³+285.9370452104x _(n) ²−223.9802206897x+70.2463717260 where the y_(n)axis is in the plane of the back shroud main inner face and the x_(n)axis is coaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) andx_(n) equals x/B₂, wherein x and y define the actual coordinates of animpeller front shroud arcuate inner face, and D₂ (which is the impelleroutside diameter) is 1560 mm, and B₂ (which is the impeller outletwidth) is 190 mm.
 48. An impeller for use in a centrifugal pump, thepump including a pump casing having a chamber therein, an inlet fordelivering material to be pumped to the chamber and an outlet fordischarging material from the chamber, the impeller being mounted forrotation within the chamber when in use about a rotation axis, theimpeller including a front shroud and a back shroud, each having a maininner face in a plane substantially at right angles to the rotation axisand a plurality of pumping vanes therebetween, each pumping vane havinga leading edge in the region of an impeller inlet and a trailing edge,wherein the back shroud further includes a nose having a curved profilewith a nose apex in the region of the rotation which extends towards thefront shroud wherein the curved profile is defined by the following:y _(n)=−52.6890959578x _(n) ⁵+79.4531495101x _(n) ⁴−45.7492175031x _(n)³+13.0713205894x _(n) ²−2.5389732284x+0.5439201928 where the y_(n) axisis in the plane of the back shroud main inner face and the x_(n) axis iscoaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) and x_(n)equals x/B₂, wherein x and y define the actual coordinates of animpeller back shroud further including a nose having a curved profile,and D₂ (which is the impeller outside diameter) is 1560 mm, and B₂(which is the impeller outlet width) is 190 mm.
 49. An impeller for usein a centrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud and a back shroud,each having a main inner face in a plane substantially at right anglesto the rotation axis and a plurality of pumping vanes therebetween, eachpumping vane having a leading edge in the region of an impeller inletand a trailing edge, wherein the front shroud has an arcuate inner facein the region of the impeller inlet, the arcuate inner face has aprofile defined by the following:y _(n)=−0.8710521204x_(n) ⁵+7.8018806610x _(n) ⁴−27.9106218350x _(n)³+50.0122747105x _(n) ²−45.1312740213x+16.9014790579 where the y_(n)axis is in the plane of the back shroud main inner face and the x_(n)axis is coaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) andx_(n) equals x/B₂, wherein x and y define the actual coordinates of animpeller front shroud arcuate inner face, and D₂(which is the impelleroutside diameter) is 712 mm, and B₂ (which is the impeller outlet width)is 82 mm.
 50. An impeller for use in a centrifugal pump, the pumpincluding a pump casing having a chamber therein, an inlet fordelivering material to be pumped to the chamber and an outlet fordischarging material from the chamber, the impeller being mounted forrotation within the chamber when in use about a rotation axis, theimpeller including a front shroud and a back shroud, each having a maininner face in a plane substantially at right angles to the rotation axisand a plurality of pumping vanes therebetween, each pumping vane havinga leading edge in the region of an impeller inlet and a trailing edge,wherein the back shroud further includes a nose having a curved profilewith a nose apex in the region of the rotation which extends towards thefront shroud wherein the curved profile is defined by the following:y _(n)=−66.6742503139x _(n) ⁵+103.3169809752x _(n) ⁴−60.6233286019x _(n)³+17.0989215719x _(n) ²−2.9560300900x+0.5424661895 where the y_(n) axisis in the plane of the back shroud main inner face and the x_(n) axis iscoaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) and x_(n),equals x/B₂, wherein x and y define the actual coordinates of animpeller back shroud further including a nose having a curved profile,and D₂ (which is the impeller outside diameter) is 712 mm, and B₂ (whichis the impeller outlet width) is 82 mm.
 51. An impeller for use in acentrifugal pump, the pump including a pump casing having a chambertherein, an inlet for delivering material to be pumped to the chamberand an outlet for discharging material from the chamber, the impellerbeing mounted for rotation within the chamber when in use about arotation axis, the impeller including a front shroud and a back shroud,each having a main inner face in a plane substantially at right anglesto the rotation axis and a plurality of pumping vanes therebetween, eachpumping vane having a leading edge in the region of an impeller inletand a trailing edge, wherein the front shroud has an arcuate inner facein the region of the impeller inlet, the arcuate inner face has aprofile defined by the following:y _(n)=−0.2556639974x _(n) ⁵+2.6009971578x _(n) ⁴−10.5476726720x _(n)³+21.4251116716x _(n) ²−21.9586498788x+9.5486465528 where the y_(n),axis is in the plane of the back shroud main inner face and the _(n),axis is coaxial with the rotation axis, and y_(n), equals y/(0.5×D₂) andx_(n) equals x/B₂, wherein x and y define the actual coordinates of animpeller front shroud arcuate inner face, and D₂ (which is the impelleroutside diameter) is 776 mm, and B₂ (which is the impeller outlet width)is 98 mm.
 52. An impeller for use in a centrifugal pump, the pumpincluding a pump casing having a chamber therein, an inlet fordelivering material to be pumped to the chamber and an outlet fordischarging material from the chamber, the impeller being mounted forrotation within the chamber when in use about a rotation axis, theimpeller including a front shroud and a back shroud, each having a maininner face in a plane substantially at right angles to the rotation axisand a plurality of pumping vanes therebetween, each pumping vane havinga leading edge in the region of an impeller inlet and a trailing edge,wherein the back shroud further includes a nose having a curved profilewith a nose apex in the region of the rotation which extends towards thefront shroud wherein the curved profile is defined by the following:y _(n)=−74.2097253182x _(n) ⁵+115.5559502836x _(n) ⁴−67.8953477381x _(n)³+19.1100516593x _(n) ²−3.2725057764x+0.5878323997 where the y_(n) axisis in the plane of the back shroud main inner face and the x_(n) axis iscoaxial with the rotation axis, and y_(n) equals y/(0.5×D₂) and x_(n),equals x/B₂, wherein x and y define the actual coordinates of animpeller back shroud further including a nose having a curved profile,and D₂ (which is the impeller outside diameter) is 776 mm, and B₂ (whichis the impeller outlet width) is 98 mm.